Solar energy is an abundant source of power for spacecraft
navigating the inner solar system. But how far away from our
star can photovoltaics work?

January
8, 2002: Â Try this: Close your eyes for a moment and
imagine the International Space Station (ISS), sunlit and gleaming
as it circles our planet.

What did it look like? The lingering image in your mind is
probably dominated by broad, beautiful wings -- the station's
awesome solar arrays.

It's no accident that solar panels dominate the station's
profile. On the ISS (as on the earth below) solar energy ultimately
powers everything that happens. Our Sun, a star named Sol, radiates
enormous power: a constant output of 4 x 1023 kilowatts
(kW), which is a 4 followed by 23 zeros! Photovoltaic
cells, which convert sunlight to electricity, need only intercept
a tiny fraction of that total to energize the station.

Above: The International Space Station in December,
2001. Credit: the crew of STS-108.

But not all spacecraft linger near Earth where sunlight is
plentiful. Many NASA probes travel far beyond our planet's orbit.
And as they do, the Sun grows more distant and dim. Somewhere
out there, solar power ceases to be a useful source of energy
for spacecraft. But where?

That's what NASA spacecraft builders want to know: Where
is the edge of sunshine?

The Space Station's solar cells, developed decades ago, convert 14% of
the Sun's energy that hits them into electricity, and modern
multibandgap cells, which convert light in multiple parts
of the spectrum into electric power, reach
efficiencies of 30% or so. Such devices work well enough in the
brightly-lit inner solar system, but more efficient cells and
larger arrays will be needed as spacecraft travel to places where
solar photons are scarce. In the outer reaches of the solar system,
for instance, the ability to convert even single photons into
electricity would be important.

"Sunlight decreases in intensity over distance by a factor
of 1/r2, where r is the distance from
the Sun," explains Geoff Landis, a scientist at NASA's Glenn
Research Center. "This means a 1-meter-square solar array
producing 400 watts at a distance of 1 AU would have to be 25
square meters in size out at Jupiter -- and almost 2,000 square
meters at Pluto to yield the same power." (Note: An astronomical
unit or "AU" is the mean distance between Earth and
the Sun. 1 AU equals 150 million kilometers.)

Left: The Sun as viewed from
distant Pluto is just another star in the night sky, albeit the
brightest one. Space artist (and space physicist) Dan
Durda
painted this
view
from the 9th planet.

Landis and his colleagues at Glenn's
Photovoltaics and Space Environment Branch are exploring new
ways to harness the Sun's power -- including more efficient solar
cells, laser-beaming energy to distant spacecraft, and solar
power systems for the Moon and Mars. "The use of solar power
is a complex field of study," says Landis. "Finding
solutions requires that we balance such factors as distance,
weight, the energy of different light bands, and the actual materials
available to us."

"Using today's technologies," he says, "the
'edge' of sunshine we can use is about four astronomical units
away from the Sun, where the sunlight is about one-sixteenth
as bright as it is near the Earth." That's beyond the orbit
of Mars (1.5 AU), but closer to the Sun than Jupiter (5.2 AU).

"With tomorrow's technologies we hope
to push that edge further out into the solar system," he
says. "Future solar collectors, for example, might use advanced
thin films -- almost like Saran Wrap -- and very lightweight
solar cells, which can roll out to an acre or more in size. Instead
of a spacecraft that carries a solar array with it, you would
have a solar array that carries a spacecraft."

Such expansive sails would also be targets
for fast-moving space-dust, so they would need to be crafted
from puncture-resistant or self-sealing materials. Yet another
challenge for spacecraft builders!

To date, the farthest any solar-powered spacecraft has ventured
from the Sun is 2.35 AU -- a record set last October by NASA's
Stardust probe. Stardust will extend its own record every day
until April, 2002, when it will reach a maximum distance from
the Sun of 2.72 AU en route to Comet Wild 2. Stardust's solar
arrays are actually producing more energy than expected, perhaps
because its photovoltaic cells operate more efficiently in the
cold of deep space than in Earth labs. No one is certain; this
is unexplored territory.

Above: (left) An artist's
impression of Stardust as it encounters Comet Wild 2 in 2004.
The craft's solar panels are prominent in this illustration.
(right) The spacecraft's orbit, courtesy of JPL's web site "Where
is Stardust Right Now?".

Not quite as far from the Sun as Stardust, NASA's experimental
spacecraft Deep Space 1 recently tested a "solar concentrator"
-- 720 lenses that focused sunlight onto 3600 solar cells. Deep
Space 1 was the first solar-powered probe to rely entirely on
triple-junction multibandgap cells. The small but innovative
system generated 2500 watts: enough to energize three microwave
ovens and more than enough to power the craft's ion engine.

Such advances will eventually propel solar power into deep
space -- perhaps out of the solar system altogether.

"In
the long term, solar arrays won't have to rely on the Sun,"
Landis said. "We're investigating the concept of using lasers
to beam photons to solar arrays. If you make a powerful-enough
laser and can aim the beam, there really isn't any edge of sunshine--
with a big enough lens, we could beam light to a space-probe
halfway to alpha-Centauri!"

Right: This artist's concept shows
a ground station beaming power to a distant spacecraft. [more]

Beaming light power to targets on Earth, in orbit, on the
Moon or on Mars and other planets -- or to distant spacecraft
-- is the stuff of science fiction. That's right up Geoff Landis'
alley. He's also a Hugo and Nebula award-winning science fiction
writer! As a scientist he and his NASA cohorts are in the business
of reaching out to the edge of sunshine every day, seeing fiction
very rapidly and certainly turning into fact.

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The Power
and Propulsion Office
at NASA Glenn Research Center (GRC) manages the development of
power technologies to meet the needs of all NASA Enterprises.